<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<style>
    iframe {
        width: 100%;
        height: 700px;
    }
</style>
<body>
    <a href="./a.html">跳转到a页面</a> 
    <a href="./b.html">跳转到b页面</a> 
    <a href="./c.html">跳转到c页面</a> 
    <hr>
    <!-- 是name不是id -->
    <iframe name="main_frame" src="./a.html" frameborder="5"></iframe>
</body>
</html>